CD123 (interleukin 3 receptor alpha, IL-3Rα) is a 40 kDa molecule that is part of the interleukin 3 receptor (IL-3R) complex. The cytokine Interleukin 3 (IL-3) drives early differentiation of multipotent stem cells into cells of the erythroid, myeloid and lymphoid progenitors. CD123 is expressed on CD34+-committed progenitors, but not by CD34+/CD38− normal hematopoietic stem cells (HSCs). CD123 is expressed by basophils, mast cells, plasmacytoid dendritic cells, some expression by monocytes, macrophages and eosinophils, and low or no expression by neutrophils and megakaryocytes. Some non-hematopoietic tissues, such as placenta, Leydig cells of the testis, certain brain cell elements and some endothelial cells, also express CD123. However, expression there is mostly cytoplasmic.
CD123 is reported to be expressed by leukemic blast cells (“leukemia blasts”) and leukemia stem cells (LSC) (Jordan et al., Leukemia 14:1777-1784, 2000; Jin et al., Blood 113:6603-6610, 2009). In human normal precursor populations, CD123 is expressed by a subset of hematopoietic progenitor cells (HPC), but not by normal HSCs. CD123 is also reportedly expressed by plasmacytoid dendritic cells (pDC) and basophils, and, to a lesser extent, monocytes and eosinophils.
CD123 has been reported to be overexpressed on malignant cells in a wide range of hematologic malignancies including acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS) (Muñoz et al., Haematologica 86(12):1261-1269, 2001). Overexpression of CD123 is associated with poorer prognosis in AML (Tettamanti et al., Br. J. Haematol. 161:389-401, 2013). AML and MDS are thought to arise in and be perpetuated by a small population of leukemic stem cells (LSCs), which are generally dormant (i.e., not rapidly dividing cells) and therefore resist cell death (apoptosis) and conventional chemotherapeutic agents. LSCs are characterized by over-expression of CD123, while CD123 is not present in the corresponding normal hematopoietic stem cell population in normal human bone marrow (Jin et al., Blood 113:6603-6610, 2009; Jordan et al., Leukemia 14:1777-1784, 2000). CD123 expression is also associated with multiple other malignancies/pre-malignancies: chronic myeloid leukemia (CML) progenitor cells (including blast crisis CML); Hodgkin's Reed Sternberg (RS) cells; transformed non-Hodgkin's lymphoma (NHL); some chronic lymphocytic leukemia (CLL) (CD11c+); a subset of acute T lymphoblastic leukemia (T-ALL) (16%, most immature, mostly adult), plasmacytoid dendritic cell (pDC) (DC2) malignancies and CD34+/CD38− myelodysplastic syndrome (MDS) marrow cell malignancies.
AML is a clonal disease characterized by the proliferation and accumulation of transformed myeloid progenitor cells in the bone marrow, which ultimately leads to hematopoietic failure. The incidence of AML increases with age, and older patients typically have worse treatment outcomes than do younger patients (Robak et al., Clin. Ther. 2:2349-2370, 2009). Unfortunately, at present, most adults with AML die from their disease.
Treatment for AML initially focuses in the induction of remission (induction therapy). Once remission is achieved, treatment shifts to focus on securing such remission (post-remission or consolidation therapy) and, in some instances, maintenance therapy. The standard remission induction paradigm for AML is chemotherapy with an anthracycline/cytarabine combination, followed by either consolidation chemotherapy, usually with higher doses of the same drugs as were used during the induction period, or human stem cell transplantation, depending on the patient's ability to tolerate intensive treatment and the likelihood of cure with chemotherapy alone (see Roboz, Curr. Opin. Oncol. 24:711-719, 2012).
Agents frequently used in induction therapy include cytarabine and the anthracyclines. Cytarabine, also known as AraC, kills cancer cells and other rapidly dividing normal cells by interfering with DNA synthesis. Side effects associated with AraC treatment include decreased resistance to infection, a result of decreased white blood cell production; bleeding, as a result of decreased platelet production; and anemia, due to a potential reduction in red blood cells. Other side effects include nausea and vomiting. Anthracyclines (e.g., daunorubicin, doxorubicin, and idarubicin) have several modes of action including inhibition of DNA and RNA synthesis, disruption of higher order structures of DNA, and production of cell damaging free oxygen radicals. The most consequential adverse effect of anthracyclines is cardiotoxicity, which considerably limits administered life-time dose and to some extent their usefulness.
Thus, unfortunately, despite substantial progress in the treatment of newly diagnosed AML, 20% to 40% of patients do not achieve remission with the standard induction chemotherapy, and 50% to 70% of patients entering a first complete remission are expected to relapse within 3 years. The optimum strategy at the time of relapse, or for patients with the resistant disease, remains uncertain. Stem cell transplantation has been established as the most effective form of antileukemic therapy in patients with AML in first or subsequent remission (Roboz, 2012).
Antibody-drug conjugates (ADC) and other cell binding agent-drug conjugates are emerging as a powerful class of anti-tumor agents with efficacy across a range of cancers. Cell binding agent-drug conjugates (such as ADCs) are commonly composed of three distinct elements: a cell-binding agent (e.g., an antibody); a linker; and a cytotoxic moiety. Conventionally, the cytotoxic drug moiety is covalently attached to lysine residues on the antibody, or to cysteine residues, obtained through reduction of interchain disulfide bonds, resulting in ADCs that are heterogeneous mixtures bearing varying numbers of drugs attached at different positions on the antibody molecule.